This section is intended to introduce various aspects of the art, which may be associated with embodiments of the invention. A list of references is provided at the end of this section and may be referred to hereinafter. This discussion, including the references, is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the invention. Accordingly, this section should be read in this light, and not necessarily as admissions of prior art.
In the oil and gas industries, data and information about subsurface reservoirs are input into physics and process based models, which are then used to build geological models, aid reservoir interpretation and characterization, and perform multi-scenario generation and uncertainty quantification. The technique becomes especially important in oil and gas industries when reservoirs of interest are formed in a confined environment or in enclosed basin-like settings.
One characteristic of fluid flow is known as a turbidity current, which can be defined as a bottom-flowing current resulting from a fluid that has higher density because it contains suspended sediment. Turbidity currents (also referred to herein as turbidity flows) are typically intermittent, but they possess considerable erosional power and transport appreciable volumes of sediment. A turbidity current is intrinsically three dimensional. In natural turbidity currents, sediments with large particle sizes, such as sand, are mostly transported in the bottom layer of the flow while sediments with smaller particle sizes such as clay and shale are transported more uniformly across the entire flow layer. This is shown in FIG. 1, which is a side elevational view of a turbidity current 10 over a solid surface such as a riverbed 11. The boundary between turbid water and clear water, higher in the fluid flow, is shown at 12. The lower region 13 of the turbidity current can be called the sandy portion because most of the sediment transported thereby has a large particle size. The upper region 14 of the turbidity current can be called the muddy portion because most of the sediment transported thereby has a smaller particle size. The amount of sediment transported by the lower and upper regions of the turbidity current is illustrated by the superimposed sediment concentration profiles 15, 16, which represent the relative concentration of mud and sand, respectively, as a function of flow depth. The sediment concentration profiles 15, 16 are used to define the boundary 17 between the upper and lower regions 12, 13 of the turbidity current and serve as a division therebetween. This division or stratification of the types of sediment transport can be easily seen from FIG. 1. Stratification has significant impacts on the flow characteristics, the interactions between the flow and the underlying topography, as well as the shape and the spatial distributions of the deposits the flow forms. The impacts are especially strong when turbidity currents occur in a confined environment. FIG. 2 depicts a cross section of a stratified turbidity current 20 in a deep water channel 21. The main flow direction is perpendicular to and flowing outward from the drawing. As clear water from above the clear/turbid water boundary 22 is entrained or incorporated into the turbidity current below boundary 22, the overall flow thickness is often greater than the depth Z of the deep water channel 21. Consequently, the turbidity current 20 spills out of the channel, as indicated by arrows 23. Because of the stratification of the muddy portion 24 and the sandy portion 25 of the turbidity current (as again demonstrated by the respective sediment concentration profiles 26 and 27), only fine grain size materials associated with muddy portion 24 of the turbidity current are transported out of the channel, while the relative coarser materials associated with sandy portion 25 are all retained in the channel. This process is known as flow stripping. If there were no stratification, or if flow models do not or cannot account for stratification, it would be predicted that the turbidity current spilling out from the channel would contain sediment from both sandy portion 25 and muddy portion 24 of the turbidity current.
While the impact of stratification on the transport and deposition of sediments in the turbidity currents is most pronounced in a confined environment where the interactions between the flow and the surrounding boundaries are the strongest, the impact is not limited to only those settings where the flow is confined. Stratification may also cause divergence of the flow directions between the sandy portion of the flow and the overall flow if there is a substantial variation of the topography underlying the turbidity current. As shown in FIGS. 1 and 2, the sandy portion of the turbidity current is often much thinner than the total depth of the turbidity current. Therefore, the flow direction of the sandy portion is much more likely than the entire turbidity current to be affected by the contours or topography of the riverbed or seabed upon which it flows. FIG. 3 is a top view of a turbidity current 30 having a flow pattern represented by curve 31. The sandy portion of the turbidity current has a flow pattern represented by curve 32. It can be seen that sandy portion 32 will follow the bottom topography, as illustrated by a series of contour lines 34, much closer than the overall turbidity current 30. The divergence of the sandy portion of the turbidity current from the overall current means that coarse, sandy materials and fine, muddy materials in the current may be transported in different directions within the same turbidity current and may be deposited or detrained in different places as well. The reservoirs formed or influenced by such divergent turbidity currents may therefore be significantly impacted with respect to compartmentalization and/or connectivity.
Process-based models that are used to aid interpretation or build geologic models of reservoirs in the deposition settings should be capable of capturing the features of the turbidity flow, such as flow stripping and flow divergence as described herein. Unfortunately, while full 3-dimensional flow models are capable of accurately computing the full 3-dimensional structures of the flow, they are computationally formidable and expensive and are not practical for use in the process-based models that are designed to simulate the formation of reservoirs with spatial scales ranging from hundreds of meters to hundreds of kilometers, and with time scales ranging from hundreds to millions of years. On the other hand, the 2-dimensional depth-averaged flow models for turbidity currently used in known process-based models are not capable of modeling flow stripping and the divergence of the bottom flow layers from the overall depth-averaged flow. Therefore, it is believed that no existing method can capture the effect of flow stripping as well as the divergence of the bottom layer flow directions from the overall flow direction, yet still be computationally efficient enough to be used in process-based models designed for large scale and long term simulations.
The foregoing discussion of need in the art is intended to be representative rather than exhaustive. A technology addressing one or more such needs, or some other related shortcoming in the field, would benefit drilling and reservoir development planning, for example, providing decisions or plans for developing a reservoir more effectively and more profitably.
Other related material may be found in the following: PCT Application WO2006/036389; Garcia and Parker, Entrainment of bed sediment into suspension, J. Hyd. Eng., 117 (4), 414-435, 1991; and Parker, G., Fukushima, Y., and Pantin, H. M., “Self-Accelerating Turbidity Currents”, J. Fluid Mech., 171, 145-181, 1986.